I have the f/4 IS and I like it a lot. Most of the points I would make have already been made. The f/2.8 is twice as heavy, the f/4 IS is much faster in handholdability while the f/2.8 is a bit faster in subject-stopping, from Bryan's ISO crops, is seems to me that the f/4 IS is sharper than the f/2.8. Really it comes down to what you want to use it for. If the sporting events you are shooting are outdoors and in the sun, then I would go f/4 IS, but if you are talking gym lighting....you will probably have trouble stopping fast action. Either way is a great choice, really.